MEXICO CITY — Gunmen killed 15 people in a mass shooting at a drug rehabilitation centre in the northern border state of Chihuahua in what was apparently a feud between drug gangs, Mexican authorities said Wednesday.

The Chihuahua state prosecutor’s office said “initial investigations prove that the attack was related to drug distribution and the feud between the Mexicles and Aztecas gangs.”

Subsequently, prosecutors’ spokesman Carlos Huerta told the Milenio television news channel that the initial death toll of 14 had risen when another shooting victim died of his wounds.

Prosecutors said at least seven other people were wounded in the attack in the state capital, also called Chihuahua.

The Aztecas are armed enforcers for the Juarez drug cartel, and they have fought longstanding turf battles with the Mexicles in the border city of Ciudad Juarez, across from El Paso, Texas.

Drug cartels have been known to use rehab centres to recruit addicts, and rival gangs sometimes assault the centres.

Huerta said initial investigations indicated that four gunmen carried out the attack late Tuesday and that some of the people at the centre may have been members of the Mexicles.

The prosecutor statement said the attackers may have also been involved in recent shootings at nightclubs in the area.

The bloodiest attack on rehab centres, of which there were several between 2010 and 2012, also occurred in Chihuahua city in 2010 and killed 19 people.

Prosecutors said they were looking at the possibility of closing the privately run rehab centre targeted in Tuesday’s attack because several incidents had occurred there.

Also Wednesday, the U.S. government offered a reward of up to $5 million for information leading to the arrest of alleged drug trafficker Fausto Isidro Meza Flores.

The FBI said in a statement that Meza Flores is the leader of a trafficking organization based in the western Mexico state of Sinaloa and was the right hand of cartel boss Alfredo Beltran Leyva before his 2008 arrest.

The U.S. government said the organization was responsible for smuggling heroin, methamphetamine, cocaine and marijuana into the United States. Meza Flores is under indictment in federal court in Washington.

Prosecutors in the central Mexico state of Puebla have opened an investigation into the killing of two municipal police officers.

In a statement Wednesday, the state prosecutor’s office said the officers were killed Tuesday near the town of Tecalcingo. One body was in the patrol car and one was behind it. They were both officers with the Tlacotepec township.